Manufacturing ultra-high strength load bearing parts using high strength/low initial yield steels through tubular hydroforming process

ABSTRACT

Rather than using a conventional stamping forming process with steels having high ultimate tensile strength and relatively low initial yield, tubular hydroforming techniques are introduced to synergize with BIW part forming, or forming of other load bearing parts. Such steels can have ultimate tensile strengths of greater than 1000 MPa and initial yields of less than 360 MPa In some embodiments, the steels have elongation of at least 40%. Such steels can include retained austenite.

PRIORITY

This application claims priority to U.S. Provisional Application Ser. No. 62/539,911, entitled “Manufacturing Ultra-High Strength BIW Parts Using Nanosteel® NXG 1200 Through Tubular Hydroforming Process,” filed on Aug. 1, 2017, the disclosure of which is incorporated by reference herein.

BACKGROUND

In automotive parts, complex geometric features are routinely designed into body in white (BIW) parts to achieve expected structural strength and stiffness or satisfy the packaging constraints. BIW parts are generally considered to be upper body, underbody and/or structural automotive components. While designers are seeking light-weighting solutions, it is always challenging to form complicated geometries with conventional advanced high strength steels (AHSS) due to their limited ductility.

Steels with high ultimate tensile strength and relatively low initial yield strength, particularly those containing retained austenite, can act as an enabler to manufacture ultra-high strength BIW parts with complex geometry, providing a high ultimate tensile strength (about 1000 MPa or greater) and superior ductility (about 40% elongation or greater). However, their relatively low initial yield strength of such steels (about 360 MPa or lower) can hinder their application to manufacturing load-bearing structural parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1d illustrate tube deformation after each manufacturing step in an embodiment of the present process.

FIG. 2 is an exemplary part made by an embodiment of the hydroforming process.

FIG. 3 is a graph showing the pre-bending pressure load history applied to an initial tube blank to form the exemplary part.

FIG. 4a-4b shows an initial tube blank for the exemplary part after prebending.

FIG. 5 shows the initial tube blank outer diameter and wall thickness prior to pre-bending.

FIG. 6 shows the tube blank outer diameter and wall thickness after the tube blank has undergone hydroforming.

FIG. 7 shows the wall thinning after hydroforming of the tube blank to form the exemplary part shown in FIG. 2.

FIG. 8 shows the true hardening stress in the finished exemplary part of FIG. 2.

FIG. 9 shows the initial tube blank outer diameter and wall thickness prior to pre-bending for a second exemplary part.

FIG. 10 shows the tube blank outer diameter and wall thickness after the tube blank has undergone hydroforming.

FIG. 11 shows the wall thinning after hydroforming of the tube blank to form the exemplary part of FIG. 9.

FIG. 12 shows the hardening stress in the finished exemplary part of FIG. 9.

DETAILED DESCRIPTION

Rather than using a conventional stamping forming process with steels having high ultimate tensile strength and relatively low initial yield strength, tubular hydroforming techniques are introduced to synergize with BIW part forming. Before undergoing the hydroforming techniques described herein, while such steels can have ultimate tensile strengths of greater than 1000 MPa, preferably greater than 1150 MPa; they have initial yields of less than 360 MPa. In some embodiments, the steels have elongation of at least about 40%. Such steels can include retained austenite. One example of such a steel is NXG 1200° steel manufactured by AK Steel Corporation, West Chester, Ohio. The methods described herein can be applied to other steels exhibiting the same or similar mechanical and hardening behaviors.

The hydroforming manufacturing process of the present embodiments comprises raw tube blanking, tube pre-bending, hydroforming (tube expansion or reduction), and trimming. In some embodiments, pre-forming and intermediate hydroforming are also used to ensure even stretching of the steel. In such an embodiment, one or both of preforming and intermediate hydroforming may occur between pre-bending and hydroforming.

In the hydroforming step, the hydraulic pressure causes the tube to expand until it matches the negative mold. This expansion introduces uniform material stretching and consequently enhances the yield strength by means of material work hardening. The enhancement of yield strength is beneficial for load-bearing structural crash performance of the formed part and enables light-weighting through the application of high ultimate tensile strength/low initial yield materials. The closed section of the hydro-formed tube also can provide stiffness and structural performance.

The present hydroforming process introduces uniform material stretching through the hydro-pressure-driven expansion and enhances the material yield strength by means of material work hardening. The enhancement of yield strength can be controlled by the amount of tube expansion, i.e., the initial tube blank diameter to the finished tube diameter according to the design specification. The initial tube blank diameter is determined by multiple factors such as initial yield strength of the steel, the targeted yield strength, and stress hardening behavior of the steel. Besides the above factors, initial and final material thickness are required to be considered to meet the final part design target. Each of these factors is known, or able to be determined, by the part designer/manufacturer.

Following are the equations, known in the industry, that can be used to determine the initial and final tube diameter and thickness:

According to Ludwik's stress hardening equations:

Y=Y ₀ +kε ^(n)  (1)

where Y₀ is the initial yield strength, Y is the targeting strength, k is the strength index, n is the strain hardening exponent.

Based on volume conservation, then

πD ₀ t ₀ =πDt  (2)

where D₀ is the initial diameter, t₀ is the initial thickness D is the final diameter, t is the final thickness.

$\begin{matrix} {{D - D_{0}} = {{\frac{t_{0} - t}{t}D} = {e_{3}D}}} & (3) \end{matrix}$

e₃ is the engineering strain at thickness direction. Then engineering strain is converted to the true strain, and Eq. (3) yields,

ln(2D−D ₀)=ln(1+e ₃)D=ε ₃ ln(D)  (4)

ε₃ is the true strain at thickness direction.

The equivalent strain can be defined as

$\begin{matrix} {ɛ = \sqrt{\frac{2}{3}\left( {ɛ_{1}^{2} + ɛ_{2}^{2} + ɛ_{3}^{2}} \right)}} & (5) \end{matrix}$

where ε₁ and ε₂ are the principal strains at tangential and axial directions respectively. Assuming the uniform expansion condition, the strain at axial direction is close to 0. Based on incompressible plasticity principal, the following relationship can be obtained

ε₁+ε₂+ε₃=0  (6)

Then substitute ε₂=0 and Eq. (5) into Eq. (6), the equivalent strain can be expressed as,

$\begin{matrix} {ɛ = {{\frac{2\sqrt{3}}{3}ɛ_{3}} = {k^{\prime}ɛ_{3}}}} & (7) \end{matrix}$

where k′ is the coefficient for the equivalent strain. Then replace the ε in Eq. (1) with above equation, we can get

$\begin{matrix} {ɛ_{3} = {\frac{1}{k^{\prime}}\left( \frac{Y - Y_{0}}{K} \right)^{1\text{/}n}}} & (8) \end{matrix}$

At last, replace the ε₃ in Eq. (4) with Eq. (4), the diameter relationship can be obtained as,

$\begin{matrix} {\frac{D_{0}}{D} = {2 - {\exp \left\lbrack {\frac{1}{k^{\prime}}\left( \frac{Y - Y_{0}}{k} \right)^{1\text{/}n}} \right\rbrack}}} & (8) \end{matrix}$

Therefore, as simplified, to estimate the desired diameter change, equation 9 below can be used:

$\begin{matrix} {\frac{D_{0}}{D} = {2 - {\exp \left( {K\left( {Y - Y_{0}} \right)}^{1\text{/}n} \right)}}} & (9) \end{matrix}$

where D₀ is the initial diameter, D is the final part nominal diameter, Y₀ is the material initial yield strength, Y is the targeting yield strength, n is the strain hardening exponent and K is the stress coefficient which determined by material stress hardening behavior and stress conditions.

This unique solution permits forming complex geometric parts with expected high strength and structural stiffness using a single material, which neither other types of advanced high strength steels (“AHSS”) nor conventional stamping can easily achieve.

The manufacturing process of hydroforming the steels of the present application comprises the following steps:

Raw tube blanking (FIG. 1a ): Select a raw tube diameter sufficient to permit stretching in the later hydroforming step thus reaching the desired yield strength, as well as keeping induced material thinning within the failure limits and design tolerance.

Pre-bending (FIG. 1b ): The raw tube is then loaded into a tube bender to generate smooth curvatures in order to achieve more uniform deformation in the later step. The goal is to create smooth curves that makes no wrinkles or large localized stress gradient.

The pre-bending is a standardized procedure for hydroforming and this requirement is a common requirement in this step.

Hydroforming (FIG. 1c ): The bent tube is placed in a hydroforming press, where the tube is filled with pressurized hydraulic liquid. The incremental pressure gradually expands the tube until it reaches the molds, and gives the part its final shape and look. In this step, the material is subjected to relatively uniform stretching, which induces the yield strength enhancement through strain hardening. The distribution of enhanced yield strength can be controlled by the amount of material stretching with selecting various initial tube diameters.

Trimming (FIG. 1d ): The formed part is taken to the cutting machine for specific trimming process.

The present process permits the exploitation of steels with high tensile strength and ductility for manufacturing ultra-high strength BIW parts. It further provides an effective light-weighting solution and enhances uniformly the material yield strength of the parts formed with the steels described herein, including NXG 1200 steel. This solution can promote the applications of steels with high ultimate tensile strengths but low initial yield strength in load-bearing BIW components, or other load bearing components. It also offers design flexibility to formed parts with complex geometric features and expected structural strength. The distribution of material yield strength can be also controlled by the amount of stretching of the steel material.

Example 1

A steel containing retained austenite is used to manufacture a front tube for an automobile, as shown in FIG. 2. Before processing, it has an ultimate tensile strength of 1150 MPa and an initial yield strength of 360 MPa. The finished part has a 20 mm outer diameter. The initial tube blank has an outer diameter of 16 mm and wall thickness of 2.0 mm.

The initial tube blank is created by tube blanking, it is then subject to pre-bending, it is then subjected to hydroforming, and then to trimming.

Pre-bending pressure is applied as shown in FIG. 3. The blank is bent in four steps, as shown in FIG. 4a , with the inner bends generating a bend with a radius of 109 mm and the outer bends generating a radius of 200 mm, as shown in FIG. 4 b.

The tube blank is hydroformed at a pressure of 500 MPa. As noted above, the initial tube blank has an outer diameter of 16 mm and a wall thickness of 2.0 mm, as shown in FIG. 5. The hydroformed part has an outer diameter of 20 mm with a wall thickness of 1.45 mm at the inner bends and a wall thickness of 1.76 mm, with an average wall thickness of 1.59 mm, as shown in FIG. 6.

No formability problems are predicted. The average wall thinning is about 20%, and ranges from a minimum thinning in the concave bending area of 12% to a maximum thinning in the convex bending area of 27%, as shown in FIG. 7.

The plastic strain ranges from 0.24, which results in a true hardening stress of 1200 MPa at the concave bending area, to 0.27, which results in a hardening stress of 1400 MPa in the flat area, to 0.37, which results in a hardening stress of 1600 MPa in the convex bending areas, all as shown in FIG. 8.

Example 2

A second tube blank was processed according to the process of Example 1. The steel contained retained austenite. Before processing, it has an ultimate tensile strength of 1150 MPa and an initial yield strength of 360 MPa. The tube blank has an outer diameter of 16 mm and a wall thickness of 2.5 mm, as shown in FIG. 9.

After prebending and hydroforming in the process described in Example 1, the outer diameter of the hydroformed tube is 20 mm, and the wall thickness varies from 1.80 mm to 2.18 mm, with an average thickness of 1.98 mm, as shown in FIG. 10.

No formability problems are predicted. The average wall thinning is 21%, with a minimum thinning of 12% in the concave bending area and a maximum thinning of 27% in the convex bending area, as shown in FIG. 11.

The plastic strain ranges from 0.25, which results in a true hardening stress of 1270 MPa at the concave bending area, to 0.28, which results in a hardening stress of 1400 MPa in the flat area, to 0.38, which results in a hardening stress of 1600 MPa in the convex bending areas, all as shown in FIG. 12.

Example 3

Steels with high ultimate tensile strength and low initial yields were formed by the following steps:

An initial tube blank is selected with an initial tube diameter sufficient to permit stretching of the steel in a later hydroforming step to reach a pre-determined yield strength while keeping induced material thinning within pre-determined failure limits and pre-determined design tolerance and forming an initial tube blank conforming to said diameter;

Pre-bending said initial tube blank to generate a smooth curvature in said tube;

Hydroforming the bent tube in a mold by filling the tube with a pressurized liquid until walls of the tube contact the mold;

Trimming the formed part.

Example 4

The process of Example 3, or any of the following Examples, wherein the steel contains retained austenite.

Example 5

The process of Examples 3 or 4, or any of the following Examples, wherein before hydroforming the steel has an ultimate tensile strength of greater than 1000 MPa and an initial yield strength of less than 360 MPa.

Example 6

The process of Examples 3, 4, or 5, or any of the following Examples, wherein before hydroforming the steel has an ultimate tensile strength of greater than 1150 MPa and an initial yield strength of less than 360 MPa.

Example 7

The process of Examples 3, 4, 5, or 6 wherein the tube blank is subject to at least one of preforming or intermediate hydroforming after pre-bending and before hydroforming. 

What is claimed is:
 1. A process for forming steels with high ultimate tensile strength and low initial yields comprising the steps of: a. Forming an initial tube blank by selecting a raw tube diameter sufficient to permit stretching of the steel in a later hydroforming step to reach a pre-determined yield strength while keeping induced material thinning within pre-determined failure limits and pre-determined design tolerance and forming an initial tube blank conforming to said diameter; b. Pre-bending said initial tube blank to generate a smooth curvature in a bent tube; c. Hydroforming the bent tube in a mold by filling the bent tube with a pressurized liquid until walls of the bent tube contact the mold to form a part; d. Trimming the formed part.
 2. The process of claim 1 wherein the steel contains retained austenite.
 3. The process of claim 1 wherein before hydroforming the steel has an ultimate tensile strength of greater than 1000 MPa and an initial yield strength of less than 360 MPa.
 4. The process of claim 3 wherein before hydroforming the steel has an ultimate tensile strength of greater than 1150 MPa.
 5. The process of claim 1 wherein the tube blank is subject to at least one of preforming or intermediate hydroforming after pre-bending and before hydroforming. 